The transverse computer assisted tomography (CT) method of calculating ventricular volumes of the human brain in vivo, using TRACE image analysis procedure was found to be highly reliable, and suitable for longitudinal studies of aging and dementia. Quantitative CT analyses in healthy aging men and women demonstrated significant sex differences in ventricular volume and in age of onset of ventricular enlargement. Structural brain changes, as measured by ventricular enlargement, and decline in cognitive performance on the WAIS appear to be relatively independent processes correlated more to the age of the subject. Three sets of monozygotic twins discordant for dementia of the Alzheimer type were studied and found to have significant differences in ventricular volumes. The Alzheimer patients also had faster rates of left ventricular enlargement than did normal controls, whereas their unaffected twins were within the normal range. Eight children with AIDS encephalopathy were studied and found to have significant reductions in the ventricular brain ratio accompanied by significant improvement of cognitive function when treated with chronic infusion AZT.